Feedstocks containing natural cresylic acid, a mixture of phenol, cresols, ethylphenols, xylenols and C.sub.9 phenols, are obtained via petroleum processing or coal processing. All such feedstocks contain neutral oil and tar bases. Neutral oil as used throughout this text is a class of organic compounds which is regarded as being comprised of impurities, and is indigenous to natural cresylic acid feedstocks. Any individual substance in neutral oil is a compound which is neither acidic, as are phenols, nor basic, such as the tar bases (pyridine and its homologues often in mixture with aniline and its homologues), but is by nature neutral in its reactions.
Spent caustic from petroleum refining is obtained by way of extraction of refinery distillates with sodium hydroxide. It contains significant amounts of cresylic acid and neutral oil impurities. According to the prior art, this spent caustic is processed by first removing sulfur compounds and then neutral oil compounds and tar bases are removed via steam distilling from the caustic solution. The cresylic acid is then "sprung" by reaction of the caustic cresylate with an acid. Finally, the sprung phenolic oil is dried by distillation to remove the water and then it is distilled to separate the cresylic acid from pitch which is a high boiling residue material. Such a process provides a raw cresylic acid mixture suitable for the present invention provided the steaming step for removal of neutral oil and tar bases is either eliminated or reduced in severity. Such a feedstock would also be suitable if dephenolized by distillation to remove phenol.
Coal processing technologies, such as coking, gasification or other coal devolatilization or beneficiation processes all yield at least two condensate streams, coal tar oil, and phenolic-rich condensate water.
These result from cooling of gaseous devolatilization products from coal. Coal tar oil may be used as a feedstock from which to obtain a distillate fraction containing cresylic acid. Coal tar oil may be distilled to separate water and light ends (the latter is often an aromatic naphtha), and also a carbolic oil distillate, also known as tar oil distillate, which contains cresylic acid. This material contains large amounts of neutral oil, often in the range of 50% to 70%, and relatively small amounts of tar bases (frequently in the 3% to 6% range). This feedstock may be processed to remove the impurities by any of a number of prior art techniques. This feedstock is a candidate material for the present invention, or this material could be dephenolized and then used as a feedstock. Once purified, including final tar base removal, the cresylic acid may be sold as such, or distilled into various products, such as cresols, or xylenols, or mixtures of these with other phenols.
Phenolic-rich condensate water (gas liquor) is most often extracted with a solvent such as an ether, ester, ketone, or a light aromatic to extract the phenols contained therein. After removal of the solvent, this mixture of phenols is often called crude phenol. Such crude phenol may be distilled to separate the cresylic acid fraction, which is the monohydric phenol fraction, from pitch. The pitch often contains dihydric phenols. The cresylic acid fraction can then be dried by distillation to remove the water and light ends which distill along with the water. The dried and depitched material (raw cresylic acid) thus obtained is far less neutral oil-rich than the tar oil distillate described above. This raw cresylic acid typically contains about 1.5% to 5% neutral oil, and 1.5% to 4% tar bases. This material can also be processed by any of a number of prior art techniques, to achieve purification, to prepare it for either sale, or fractionation and then sale. It is also a candidate material for the present invention either as such, or dephenolized.
The tar oil distillate feedstocks have in the past been purified by way of any of three types of process technologies. These categories are (1) causticizing, usually with sodium hydroxide, and steam distilling the neutral oil and tar bases, followed by springing the wet phenolic oil, (2) any of various single solvent approaches to the task of extraction of cresylic acid from neutral oil, and (3) any of numerous dual solvent technologies, using a two phase immiscible pair of solvents, one to dissolve the phenols, and another to dissolve the neutral oil, followed by solvent recovery systems for separation of the solvents from the product phenols, and from the neutral oil. Additionally, the Phenoraffin process is suitably classified as a dual solvent system, but it is a hybrid in the sense that it employs sodium ion in the form of sodium cresylate as one of the solvents, and in the sense that some of the neutral oil is removed in a distillation step to recover toluene.
The first of the above three categories is the oldest of the cresylic acid processing methods. After causticizing the feedstock (not needed in the case of spent caustic which is already causticized), the caustic phase is decanted away from any insoluble neutral oil. In the case of spent caustic, there is no floating neutral oil since the spent caustic is decanted at the refinery and, in the case of crude phenol extract from gas liquor or the like, there is not enough neutral oil present to form a neutral oil phase. The next step consists of steam distilling the neutrals and tar bases dissolved in the caustic cresylate. Following this, an acid is used to spring the wet phenolic oil. The wet sprung cresylic acid is next dried by distilling away the water. When required, the cresylic acid may be distilled to separate it from materials which are pitch-like, or a small amount of sulfuric acid may be added and this mixture distilled, to separate the cresylic acid from traces of tar bases not thoroughly steamed out, as well as from pitch-like materials.
This prior technology, particularly if sodium hydroxide must be purchased or regenerated, is costly, and also quite bulky and energy intensive. If caustic is not regenerated from process byproduct sodium carbonate (obtained via carbon dioxide springing of the wet phenolic oil) by way of quicklime, then the sodium-containing waste stream must be transported and sold.
The single solvent extraction methods employ a polar solvent to extract the phenols from tar oil distillates of various kinds. These methods employed aqueous solutions of a number of solvents, such as glycols, ethanolamine, ammonia, acetic acid, ethylamine, sodium salicylate, methanol, or even hot water. Distillation was typically used to separate the solvent from the extract for recycle. All of these methods provided cresylic acid containing too much neutral oil to be salable in today's market.
Because of the purity vs. yield problem with the single solvent approaches, dual solvent systems were developed, utilizing fractional, side feed, countercurrent extraction as opposed to ordinary countercurrent extraction. These processes utilized a polar solvent to dissolve the cresylic acid and a non-polar solvent to dissolve the neutral oil. The side feed fractional aspect of these methods permitted a number of liquid/liquid equilibrium stages to be used in the process to separate one component of a feedstock from another. This granted the ability to exercise control over both purity (the number of stages employed in the direction of removal of the heavy solvent), and recovery (yield of cresylic acid by way of the number of stages in the direction of removal of the light solvent). Included among the polar solvents used have been aqueous solutions of methanol, ammonia, acetamide, acetic acid, ethanol, glycols, monoethylamine and sodium salts of sulfonic acids. Non-aqueous polar solvents have included glycols of various kinds, and glycerol. Light, non-polar solvents have included hexane, heptane, octane, petroleum ether, diesel, and various non-aromatic naphthas. Distillation was generally used to separate the solvents from the extract and the raffinate to ready them for recycle and to isolate the phenolic product and neutral oil streams.
The best known of the dual solvent fractional extraction methods is that disclosed in U.S. Pat. No. 2,666,796 which uses aqueous methanol and hexane. The literature available from the 1950's regarding this process shows that neutral oil contents of about 0.2% or even 0.1%, are possible. It must be said, though, that the analytical methods used at that time are far from adequate to reveal all of the neutral oil actually present in a cresylic acid material.
The earlier methods of neutral oil analysis were based on causticizing a sample of cresylic acid and steam distilling the neutral oil and tar bases from the sample. Following this, the tar bases were titrated, and finally the volume of neutral oil present in the distillate was measured and multiplied by a specific gravity factor to arrive at a weight percent neutral oil. This method is now out-dated. It is not capable of revealing very much of the ketone content (especially the cyclopentenone homologues), or the content of nitriles (much of the ketone content is destroyed by the action of boiling caustic, as well as most of the benzonitrile), nor will it reveal the most polar among the neutral oil substances that are inclined to dissolve in the steam distillate water phase. The modern technique employs a number of methods, each designed to reveal a particular class of neutral oil substances or an individual substance, to arrive at an accurate total neutral content for a sample.
The cresylic acid product of the subject invention, derived from tar oil distillate, does not reveal any neutral oil at all by the old method of measuring the volume of oil in a steam distillate even though the water phase can exhibit a slight haziness. Thus, by the older methods, this product would be said to have nil, or zero neutral oil content, not enough to form a detectable amount of oil to measure an oil volume. Using a tar oil distillate as a feedstock to the subject invention, it has been found that the neutral oil contents of products of various dual solvent methods of prior art, as determined by modern methods of analysis, are roughly an order of magnitude greater than the results reported from steam distillation of a causticized sample using the old measuring techniques. It is very difficult to even force the prior art process to the point where cresylic acid products have less than 1% neutral oil content as analyzed using modern analytical methods.
One of the problems of prior art methods which employed aqueous, light boiling solvents is that after removal by distillation of the solvent such as methanol, the bottoms product is two phases, wet cresylic acid product and water. Decanting the water from the wet cresylic acid is troublesome at best. The water phase coalesces and separates from wet cresylic acid with difficulty. Once the water is coalesced, and the wet cresylic acid is decanted away from it, the residual dissolved water must be distilled from the cresylic acid and this is quite energy intensive. Thus, there has been an incentive to use dry solvents to avoid these problems.
Several investigations have been done, using triethylene glycol as the extractant, and alternately, using glycerol as the extractant. A paper describing such studies appears in the Journal of Applied Chemistry, June, 1952, p.314. Ternary solubility diagrams were provided for the systems phenol, triethylene glycol and hydrocarbon (a coal tar hydrocarbon distillate fraction), and also for phenol, glycerol and hydrocarbon, at three temperatures. Additionally, the effect of several amounts of added water to the systems was defined in solubility diagrams. Results showed that the effect of water added to the triethylene glycol system was favorable, indicating that limited addition of water to triethylene glycol provided an improved product, containing less neutral oil. The effect of water upon glycerol was unfavorable, though, as the glycerol phase in the presence of added water tended to dissolve more of the neutral oil hydrocarbon than if the glycerol were anhydrous.
Another paper, published in March of 1953, also in the Journal of Applied Chemistry reports a similar study excepting that the system studied was cresylic acid, triethylene glycol or glycerol, and a hydrocarbon distillate derived from coal. The solubility of the mixed phenols in the glycerol was quite low, so much so that a large number of stages would be required to achieve good recovery from the raffinate. Also, the neutral oil solubility in the glycerol phase was greater. Thus, the separation was not as desirable as in the above mentioned system using phenol, rather than mixed phenols. In other words, the phenolics were not adequately soluble in the glycerol and the neutral oils were more soluble than in the earlier study. It was concluded that glycerol was not recommended for the extraction of mixed phenols. For triethylene glycol, the solubility of the phenolics was good using triethylene glycol containing 37% water. It was reported that it would be possible to obtain a product containing 0.1% neutral oil, but the actual content would probably be determined to be much higher using modern analytical methods.
In the development of the present invention, studies of anhydrous triethylene glycol have shown that even using modern high efficiency fractional countercurrent extraction equipment such as the Karr column having approximately seven to eight equilibrium stages, it was not possible to achieve a total neutral oil content in cresylic acid products less than the 1% to 2% range, as analyzed using modern analytical techniques. The studies with tar oil distillate feedstock have shown that the well known wet methanol/hexane system, using modern extraction equipment and analytical methods, also provides cresylic acid containing neutral oil in the 1% to 2% range, at best.
If one were to use aqueous triethylene glycol, it would be mandatory to distill all of the water to separate it from the product cresylic acid since, unlike wet methanol, the glycol has a boiling point greater than that of the cresylic acid. After methanol has been removed by distillation, it is possible to decant water from cresylic acid since a sizable fraction of the water then forms a separate phase. No comparable process option exists with triethylene glycol. Distillation of water is very energy intensive and therefore quite uneconomic on a commercial basis.
The work mentioned above appearing in the Journal of Applied Chemistry eventuated in U.S. Pat. No. 2,790,834 and British Patent 730,473. As in the published papers, the preferred heavy extraction solvent employed was either an aqueous solution of triethylene glycol, or alternately glycerol which was said to be less desirable for extraction of mixed phenols for reason of high solvent ratio and increased number of extraction stages required to extract the phenolics. Other polyhydric alcohol solvents were also discussed.
The literature of Cumming and Morton indicated that they found glycerol to be inadequately capable of extracting mixed phenols (cresylic acid) and this is supported by tests leading up to the present invention. In terms of polarity and degree to which hydrogen bonding occurs, only water exceeds glycerol, in data tables listing solvents which are industrially available.
Bench scale studies with glycerol revealed that this solvent is very nearly incapable of extracting cresylic acid from tar oil distillate; so little of the cresylic acid would partition into glycerol that the glycerol remained very nearly colorless. As noted above, triethylene glycol, in an anhydrous state, performed quite well in terms of its ability to extract the cresylic acid from an aromatic neutral oil matrix, so well, in fact, that it was insufficiently capable of rejecting the most difficult of the various neutral oil substances such as the ketones, the naphthalenes and the nitrites. Triethylene glycol can be used in an anhydrous state, and this is a very desirable aspect of its use, but the purity of the products achieved with it is at least an order or magnitude, if not twice an order, worse than desired.
Upon first investigating triethylene glycol (dry) with a Karr column, using tar oil distillate, it was discovered that nitrites, similar to the performance of the wet methanol, were poorly removed, and the content of naphthalene and its homologues was found to be significantly worse than when using aqueous methanol. The ketones were a little more readily removed. The tar oil distillate used for all of these studies contained about 60% neutral oil.
The studies of the aqueous methanol/hexane approach to neutral oil removal, using a Karr column, or alternatively a mixer settler arrangement, revealed that this solvent system is fairly effective at separating naphthalene and its homologues from cresylic acid (products from tar oil distillate can be obtained at less than 100 ppm naphthalenes), but quite poor in separation of ketones (products range from 1% to 3.5%) and nitriles (products at best contain about 1000 ppm total nitrites).
To simultaneously obtain high product recovery and high product purity has not been possible via any of the liquid/liquid countercurrent fractional extraction techniques of prior art that have been studied using identical feedstocks.